In recent years, progress of electronic devices has been remarkable, and reduction in size and weight of portable electronic devices has been in progress at a rapid pace. Accordingly, batteries to be used as their power sources are also required to have a high energy density so that they can be made small in size and light in weight. Particularly, as a non-aqueous secondary battery, attention has been drawn to a lithium ion secondary battery.
A lithium ion secondary battery is constituted usually by components such as a positive electrode, a negative electrode, a non-aqueous electrolyte and a separator. Among them, the positive electrode is obtained usually by dispersing e.g. a cathode active material and an electrically conductive material in an organic solvent or water together with a binder, to prepare a positive electrode material mixture, which is then applied to the surface of a current collector, followed by evaporation of the solvent to fix the electrode active material to the electrode surface. If the binder fails to fix an adequate amount of the electrode active material to the electrode, a battery having a large initial capacity cannot be obtained, or the electrode active material falls off from the electrode by repetition of charge and discharge, whereby the capacity of the battery decreases.
As the positive electrode material mixture, an organic solvent type positive electrode material mixture or an aqueous positive electrode material mixture is obtainable by using an organic solvent type binder or an aqueous binder. Recently, attention has been drawn particularly to a positive electrode material mixture employing an aqueous binder, since it is thereby possible to reduce the production costs for an electrode by reducing use of an organic solvent and to improve the operation environment by reducing the environmental load.
As an organic solvent type binder, one having polyvinylidene fluoride (PVDF) dissolved in an organic solvent such as N-methylpyrrolidone is widely used (e.g. Patent Document 1). Further, as other binders having bonding properties or flexibility improved, polymers having vinylidene fluoride copolymerized with hexafluoropropylene, chlorotrifluoroethylene, tetrafluoroethylene or the like, are known (Patent Document 2).
As an aqueous binder, a latex of styrene/butadiene copolymer rubber or polytetrafluoroethylene produced by an emulsion polymerization method is known. However, polytetrafluoroethylene is poor in adhesion to a current collector and has had a problem such that when an external force is exerted, for example, in a winding up step, a cathode active material is likely to peel off from a metal foil as a current collector, or when used as a battery, the stability in long term charge/discharge cycles tends to decrease. Whereas, styrene/butadiene copolymer rubber is a rubber polymer and thus is excellent in flexibility and adhesion as compared with polyvinylidene fluoride or polytetrafluoroethylene, but oxidation resistance of the polymer is poor, and especially when the charging voltage is increased, its charge/discharge durability is inadequate.
On the other hand, with respect to a cathode active material, lithium cobaltate has heretofore been used in many cases, but various developments have been in progress from the viewpoint of the battery properties and safety, costs, etc. Particularly, a so-called three elements type cathode active material having the capacity increased by reducing highly costly cobalt as far as possible and replacing it with nickel, manganese or the like, has attracted attention as a positive electrode material for the next generation of lithium ion batteries. However, such a cathode active material containing a lithium atom has a high basicity and thus has a problem such that when a solution of a polymer having vinylidene fluoride polymerized is used as a binder, it is likely to be reacted with the polymer itself so that the positive electrode material mixture tends to be gelled. In order to solve such a problem, in an organic solvent type binder, a study has been made to reduce the basicity of the positive electrode material mixture by reducing water contained in the binder as far as possible or by using, as a cathode active material, one having a low basicity only (e.g. Patent Document 3). However, in a case where an aqueous binder containing water as the medium is to be used, it is unavoidable for a positive electrode material mixture to be basic, and accordingly, it has been desired to provide a positive electrode material mixture which is less likely to be gelled and which has high adhesion to a current collector, even in a system where a highly basic active material is used and further, to provide an electrode having excellent battery properties.